1. Field of the Invention
The present invention generally relates to a plasma-arc cutting apparatus and more particularly to such an apparatus capable of deflecting a plasma arc by utilizing a magnetic field.
2. Related Art Statement
There is known a plasma-arc cutting apparatus having a plasma torch for ejecting a plasma arc toward local portions of a metallic or non-metallic workpiece in the form of, for example, a plate, and cutting the workpiece by fusing or melting the local portions thereof.
However, a desired part cut from a workpiece by a plasma-arc cutting apparatus adapted to apply a plasma arc normally to the workpiece, suffers from a comparatively large bevel of the cut surface thereof with respect to the upper or lower surface of the workpiece (or desired part), because the upper portion of the workpiece is more melted than the lower portion. In other words, the cut surface of the desired part is not made normal to the upper or lower surface thereof. Thus, additional work steps are needed for finishing the cut surface, namely, processing the cut part so as to have a cut surface normal to the upper or lower surface thereof (hereinafter, this surface being referred to as the "normal" cut surface).
In the above-indicated situation there is known the technique of deflecting a plasma arc ejected from a plasma torch, by applying a magnetic field thereto. Japanese Patent Application laid open under Publication No. 49-13062 (1974) discloses the method of applying a magnetic field to a plasma arc ejected from a plasma torch, and thereby deflecting the plasma arc toward a desired part to be cut out of a workpiece, so that the desired part has a normal cut surface.
The plasma-arc cutting technique taught by Japanese Patent Application No. 49-13062 can provide a part having a normal cut surface in the event that the part is cut from a workpiece as the plasma torch is moved in a pre-selected cutting direction at a constant cutting speed. Since the direction of deflection of the plasma arc is not changed during the cutting operation, however, the known cutting technique cannot provide a part having a normal cut surface over an entire length thereof in the event that the part is cut in various directions different from the above-indicated pre-selected cutting direction, for example at a curved portion or a sharp corner of the part, or in the event that the part is cut at different cutting speeds during the cutting operation.